The present invention relates to a process for producing a porous ceramic matrix, for catalyst materials and like purposes.
It is one object of the invention to provide an efficient method for forming a porous ceramic matrix capable of different industrial uses.
EP-A-0353669 discloses a method of forming an article by pressing a free-running mixture of water, a ceramic powder, combustible organic substances (coconut shell granulate and/or cellulose granulate), a binder and optionally a lubricant, drying the pressed body and firing to decompose the organic substances.
EP-A-0767154 discloses a method for forming solid, homogenous or porous shaped bodies from a powder selected from metals, intermetals, hard metals, cermets and/or ceramics comprising the following steps:
a) preparing of an aqueous slurry of the powder and a protein substance which is soluble in water at ambient temperature and capable of forming an irreversible gel on heating to a temperature of 90xc2x0 C.;
b) forming the slurry into a body of the desired shape by moulding, tape casting or drop forming; and
c) heating to a temperature at least corresponding to the gelling temperature of the protein.
The content of the protein is suitably 10 to 15% by weight based on the aqueous phase which makes the process of dispersion difficult and limits the amount of solids which can be dispersed. In examples 1, 2 and 3 of this document, the content of solids is 32 vol. %, 20 vol. % and 37 vol. % respectively.
According to the invention in one aspect there is provided a method of making a porous ceramic matrix, the method comprising:
i) forming a dispersion comprising water, refractory particles, at least one pore forming organic material and a polymeric dispersant;
ii) adding a water-sensitive cyclic ester to the dispersion in a concentration of from 1 to 10xc3x9710xe2x88x925 mole/gm of dispersion;
ii) placing the dispersion in a mould;
iv) allowing or causing the cyclic ester to be hydrolysed to cause an increase in the viscosity of the mix thereby to cause it to solidify and form a green body;
v) removing the green body from the mould; and
vi) heating the green body to remove water and burn away the organic material.
In the examples which follow the concentration of cyclic ester is always less than 1% by weight and the concentration of solids is always above 50 vol. %.
The refractory particles may be selected from a wide variety of metals or non-metals. The particles may be so-called low surface area, medium and high surface area materials. These are exemplified respectively by values of about 0.1 to about 20 m2/g; about 20 to about 200 m2/g; and about 200 to about 10002/g. Medium surface area materials include transition aluminas, kieselguhr and activated clays. Examples of low surface area particles are alpha alumina, mullite, cordierite, fused silica, zirconia, titania; camma alumina some carbon and silica and many alumina hydrates such as boehmite, gibbsite, and bayerite are representative of high surface area materials. Other materials are copper, zinc, hydroxycarbonates; metal oxides such as nickel oxide (which can be reduced to the metal to provide nickel sponge); aluminosilicates, silicon carbide, silicon nitride; and the like.
The polymeric dispersant may be any substance suitable for the purpose. Examples include acrylates, polyacrylates and the like; one specific example is ammonium polyacrylate.
Preferably the pore-forming material is particulate, the dimensions being chosen to determine the size of the pores. Preferably the particles are of different sizes and shapes, so that interconnections are more easily made. The pore-forming material may be selected from a wide variety of natural or-synthetic organic substances. Examples include carbon fibre or powder; almond shelf, coconut shell particles, olivestone powder, wood flour, chopped cotton fibres; chopped fibres of polyester, nylon, polymer beads, stearates, extrudates, granules of polystyrene or polypropylene; or the like. The content and nature of the pore-forming material may be used to control porosity and pore size distribution in the matrix.
To be suitable for the purposes of this invention the water sensitive cyclic ester must be one which will form a substantially homogenous dispersion with the other ingredients and will hydrolyse in a reasonable time period to form a shaped article which is sufficiently rigid in the green state to be de-moulded from the mould.
The rate of hydrolysis is selected so that a demouldable composition is formed within say from 10 minutes to a few hours. If it is too slow the technique is not acceptable industrially, and if it is too fast the composition will set during casting and defects will form inside the green body.
The water sensitive cyclic ester may be selected from a variety of substances. Preferably the ester is a cyclic ester, examples being carboxylic lactone and lactide. Suitable candidates for the present invention include:
very fast reacting agents such as delta-glucono-lactone, 3,6-dimethyl-1, 4-dioxane-3,5-dione and other 6-ring-membered lactones and lactides;
fast reacting agents such as: alpha-D-glucoheptonic gamma lactone, D-glucorono-6,3-lactone, L-gulonic-gamma lactone, D-gulono-1,4-lactone, L-mannonic-gamma lactone, D-erythronic-gamma lactone, alpha-hydroxy-gamma-butyrolactone, delta-valerolactone, alpha-angelicalactone, and other 5-ring-membered lactones with no more than three Methyl side-groups;
medium rate reacting agents such as: beta-butyrolactone and other 4-ring-membered lactones; and
slow reacting agents such as: pantolactone, epsilon-caprolactone, gamma-butyrolactone, gamma-valerolactone, and other high ring-membered lactones and lactones with more than three methyl side groups.
Typically the concentration of the agent is about 1 to 10xc3x9710xe2x88x925 mole per gram of the dispersion, preferably 2-5xc3x9710xe2x88x925 mole per gram of the dispersion. Increasing concentrations will shorten the period before the composition has set.
Preferably the temperature of the mix is reduced below ambient to slow the rate of hydrolysis and thereby improve the homogeneity of the suspension on addition of the agent. Upon the addition of the destabilising agent, the viscosity starts to rise. At lower temperatures the rate of viscosity increase is much lower because the kinetics of the reaction are lowered. Working at a lower temperature (not too low otherwise the water present in the system will freeze) would allow a much longer time in which to mix in the agent. A preferred temperature range is between about 0xc2x0 C. and 20xc2x0 C., preferably between 0xc2x0 C. and 10xc2x0 C. before the addition of the destabilising agent. A mixing stage may result in the entrapment of air. If needed, therefore, the method may include the step of applying a vacuum to the composition to remove air bubbles introduced by mixing.